Type-1 diabetes (T1D) results from the destruction of insulin-producing p-cells driven by autoreactive T lymphocytes. In both mice and humans, T1D is a regulated disease, with important checkpoints: at the initiation of autoimmunity when inflammatory cells enter the lesion, resulting in insulitis; later, in the consequences of this insulitis, which can persist for long periods, or conversely result in p-cell destruction in short order. The cellular dynamics of the insulitis are poorly understood, as are the regulatory mechanisms that prevent autoreactive cells from wreaking terminal damage. We propose to analyze the equilibrium between autoimmune effector cells (Teff) and regulatory cells (Treg) using the powerful imaging technologies implemented by our MGH colleagues (Projectl, CoreA). The studies will use the NOD model of T1D and transgenic derivatives thereof, with mice in which cells are labeled by transgenic expression of fluorescent proteins, in pancreatic p-cells or in T cell subpopulations. They revolve around three questions: 1. Is the organized insulitis self-sustaining, when and by which route are new effector and regulatory T cells recruited? Basic aspects of the population kinetics of insulitis components will first be analyzed by flow cytometry and conventional microscopy. We will then use intravital microscopy to track the path to the islets of autoreactive Teff or Treg cells. Mesoscopic Fluorescence Tomography (MFT) will be used to image the global distribution of T cells in the pancreas, collaborating with Project2 for the combined analysis of inflammatory and p-cells. We will also attempt to quantitate dynamically the influx of inflammatory cells over the course of the disease by non-invasive Fluorescent Protein Tomography (FPT) in vivo. 2. How are population dynamics affected by the interplay between effector and regulatory cells? The impact of Treg cells on autoimmune diabetes is well established, but there is significant debate as to where and how Tregs interfere with autoimmune pathogenesis. Using the same imaging tools, we will analyze the population dynamics of insulitic lesions in conditions where Tregs vary (in particular with time- controlled Treg cell ablation), and use 2-photon intravital microscopy to visualize the interactions between Teff, Tregs, and islets DCs or p-cells. 3. When are pro-inflammatory or regulatory transcriptional cytokine programs acquired? We will backcross reporter transgenes encoding fluorescent proteins under the control of the IFNy, IL17 and IL10 loci, and use MFT to visualize and quantitate the evolution of the activity of these key cytokine loci, over the course of unfolding diabetogenesis in NOD mice. These investigations will use cutting edge imaging tools to visualize the autoimmune attack of insulin- producing cells in autoimmune diabetes. They will deepen our understanding of the causes of diabetes and open several avenues for diagnostic or prognostic imaging in at-risk individuals, and for treatment targets.